1. Field of the Invention
The present invention relates generally to the generation of communication signals and, more particularly, to the generation of nonconstant-envelope communication signals.
2. Description of the Related Art
A variety of communication systems modulate information signals onto radio frequency carrier signals for transmission between system users. Because the information signals are often baseband signals, the modulated carrier signals must be generated with some type of frequency upconversion that translates lower frequency signals to higher frequency signals. In one conventional upconversion method, communication signals are converted in a nonlinear mixing process with local-oscillator signals to produce sum and difference signals. After this mixing process, filters are used to select a desired sum signal which is then amplified to form a transmit signal.
Although this upconversion method is effective, it requires space to house the necessary circuit elements (e.g., mixers, local oscillators, amplifiers and filters) and substantial power to operate these elements. Because space and power are severely limited in a number of communication devices (e.g., in handheld radio telephones), a need exists for an upconversion method that reduces the demand on space and power.
This demand is often met with a phase-locked loop which is a closed-loop system in which a voltage-controlled oscillator directly generates the transmit signal. In response to a modulated signal and a sample of the transmit signal, a phase detector generates an error signal which is coupled to control the voltage-controlled oscillator through a loop filter that establishes a loop bandwidth. The modulation is thereby processed onto the transmit signal which is simultaneously upconverted because the sample signal is typically passed through a frequency divider before it is applied to the phase detector. Accordingly, phase-locked loops are used in a variety of radio transmitters because they provide the necessary upconversion process while providing significant advantages (e.g., reduced parts count, space and power drain).
However, upconversion via phase-locked loops has generally been limited to constant-envelope modulations (e.g., gaussian minimum shift keying (GMSK)). Various nonconstant-envelope modulations (e.g., shifted M-ary phase shift keying (PSK)) can provide improved communication performance (e.g., higher data rates) but their envelope information is not faithfully processed through phase-locked loops because the output signal of a voltage-controlled oscillator is substantially constant over its operating band. In addition, the loop bandwidth is typically limited to provide loop stability and to reduce the noise that is processed onto the transmit signal. Therefore, phase-locked loops also generally fail to faithfully reproduce the phase information of nonconstant-envelope modulations.
Various modifications have been proposed to improve amplitude and phase information processing through phase-locked loops to thereby improve their performance with nonconstant-envelope modulations but these modifications have typically been complex. Accordingly, they impose penalties (e.g., increased parts count, power drain and space requirement) that substantially negate the upconversion advantages of phase-locked loops.
The present invention is directed to phase-locked loop methods and structures for generating modulated communication signals with nonconstant envelopes. These methods and structures realize the improved communication performance of nonconstant-envelope modulations (e.g., higher data rates) with the upconversion advantages of phase-locked loops (e.g., reduced parts count, space and power drain).
Transmitters of the invention augment a phase-locked loop with first and second feedforward paths that substantially restore phase and amplitude information to a transmit signal that is generated by a voltage-controlled oscillator of the phase-locked loop. The first feedforward path includes a detector that extracts a phase-correction signal from a modulated intermediate-frequency signal and the gain of this path is adjusted to realize a path transfer function of s/Kv wherein the voltage-controlled oscillator has a transfer function of Kv/s. The second feedforward path includes another detector that extracts an envelope-correction signal from the modulated intermediate-frequency signal and a variable-gain output amplifier is arranged to amplify the transmit signal with a gain that responds to the envelope-correction signal.
The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings.